The overall aim of research is to increase understanding of the vocal communication of non-human primates. This is important both for a general understanding of animal communication and because research on non-human primates can clarify ways in which these species may potentially serve as animal models for the study of human speech. Our specific focus is vocal development. Subjects are captive rhesus and Japanese macaques, housed in six social groups that accurately reflect the two species' social organization under natural condition. Over the next three years we will use observational sampling, tape- recording, and computer-based acoustic analysis to document the normal course of vocal development in each species. Simultaneously, we will cross-foster infants between species, thereby exposing developing individuals to an auditory environment that is critically different from the one they would normally experience. Two infants - one from each species - have already been successfully cross-fostered, indicating that our methods are feasible and likely to provide important new data on non-human primate vocal development. If cross-fostered infants conform to their adopted species' pattern of vocalizations, this would provide the first strong evidence of environmentally- induced vocal modifiability in primates. Conversely, evidence that infants follow their own species' pattern would indicate a fundamental difference in vocal production between human and non-human primates.